While many 9/11 ‘truthers’ have claimed the terror attack was an inside job over the past two decades, a number of other mysteries – while less controversial – remain unsolved.

Here are some of the enduring mysteries of September 11.

Mystery of ‘The Falling Man’ who plunged from World Trade Centre on 9/11

The so-called “Falling Man” was snapped by Associated Press photographer Richard Drew as he fell from the North Tower on September 11, 2001 – yet 20 years on, we still don’t know his name.

Taken at 9.41am, the chilling image showed a man plunging head-first towards the ground, his legs pointed skyward, and the steel exterior of the North Tower in the background.

Many attempts have been made to identify the Falling Man over the last 20 years, but none have so far proved successful.

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Mystery of ‘The Falling Man’ who plunged from World Trade Centre on 9/11
The photo of the mystery man was taken by Associated Press photographer Richard Drew.

What really happened on Flight 93?

When United Airlines Flight 93 was hijacked as part of the September 11 attacks, it is believed the four Al-Qaeda terrorists intended to crash it into the US Capitol building.

Instead, following a passenger revolt, the aircraft crashed into a field in Pennsylvania well short of its target.

All 44 people onboard – including the four hijackers – were killed, and the story of the passengers’ heroic struggle rightly inspired movies and television dramas.

Nonetheless, the narrative that those onboard managed to wrestle control of the jet and crash it themselves, which has gone down in legend, is actually less clear.

Based on the records of telephone calls made from the plane, it’s certain that many passengers intended to storm the cockpit, but no evidence has yet come forward proving they did.

According to the Independent, despite the cockpit voice-recorder being retrieved in perfect working order, its contents are yet to be published.

Who was behind deadly anthrax attacks?

A week after the September 11 tragedy, the US was rocked by another, lesser known attack – this time through deadly anthrax.

Over a series of weeks, letters containing anthrax spores were mailed to news media offices and to Democratic senators Tom Daschle and Patrick Leahy.

Five people were killed and 17 infected, leading the FBI to launch an investigation that became “one of the largest and most complex in the history of law enforcement”.

Despite this claim, it was never fully proven exactly who was behind the attacks.

While initially linked to Al-Qaeda or Saddam Hussein in Iraq, forensics showed the weapon was made on US soil.

Why wasn’t there more warning about toxic fallout?

Following the search for survivors in the rubble of the Twin Towers, evidence mounted that dust from the fallout had a horrific impact on rescue workers’ health.

In the years since the disaster, research has shown that male firefighter responders had a cancer rate 19 per cent higher than those not exposed, alongside higher rates of asthma and PTSD.

In the new ITV documentary, 9/11: Under Attack, a shell-shocked pair of workers who had managed to escape from the building are shown covered in white dust – save from a small patch on their faces where someone had mercifully poured water.

“Whatever this stuff is, I hope it doesn’t kill us,” said the woman.

But many of the people who survived the attacks would go on to become extremely ill as a result of breathing in the dangerous air.

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The ultimate target of Flight 93?

The ultimate target of Flight 93 remains one of the biggest mysteries of 9/11.

The authoritative 9/11 Commission report notes that the hijackers had turned the navigation to guide the Boeing 757 towards Washington. But the report is inconclusive about the final destination: The objective of Ziad Jarrah, the pilot among the hijackers, “was to crash his airliner into symbols of the American Republic, the Capitol or the White House.”

How did the towers collapse so quickly?

Engineers designed the Twin Towers to withstand the impact of a commercial jet airplane. Yet, both towers collapsed within two hours of being hit. Could the towers have been rigged with explosives (demolition-style) prior to the planes hitting?

Snopes discredited this as not scientifically sound, and the world may never know how these optimally-engineered towers collapsed so quickly.

How did the twin towers collapse so quickly on 9/11?
How did the twin towers collapse so quickly on 9/11?

Did the aircraft materials contribute?

The primary material used in constructing airplanes is aluminum. According to the Smithsonian channel, two scientists have independently pointed out that at extremely high temperatures (over 1,000°F), aluminum melts and becomes a powerful explosive when it comes into contact with water. They theorize the burning towers formed a “furnace” of sorts, melting the aluminum, which would have then exploded on contact with water from the building’s sprinklers.

This is one explanation that could account for the towers’ “demolition-style” collapse.

Why did the government ignore the warning the day before?

The day before the 9/11 attacks, the U.S. government’s National Security Agency (NSA) intercepted two communications, both from Afghanistan to Saudi Arabia. One says, “Tomorrow is zero hour.” The other says, “The match begins tomorrow.” These messages were not translated until September 12, according to CNN. Was there a reason for this?

This remains a mystery. Here are 16 more of history’s strangest unsolved mysteries.

Where was the NSA on September 10, 2001?

Assuming the NSA didn’t translate these ominous messages on the day they were intercepted, there’s still the question of what the agency was doing that day.

The CIA had been on high alert since August 23 that a terrorist attack by Al Qaeda was imminent. Check out these secret U.S. Government operations, revealed.

Pedestrians running from the scene as one of the towers collapses on 9/11

Sneha Philip: The mystery of the woman who disappeared on 9/11

Twenty years have passed since Sneha Anne Philip was last seen alive in New York City.

Within that time, she’s been included, removed and readded to the official 9/11 victims list, accounts of her actions have been given and retracted, and even her official date of death has been changed. The descriptions of her life and character given by police and her family are so markedly different, they could be about two entirely separate people.

In other words, even after after two decades, the fate of Sneha Philip remains a complete mystery.

The 31-year-old doctor spent a seemingly peaceful afternoon at home alone in her downtown Manhattan apartment on 10 September, 2001. She repotted some orchids, and spent two hours chatting to her mum over instant messenger, telling her about a fun night out she’d had and sharing plans for the week ahead. In the late afternoon, she headed out to do some shopping at a nearby department store, where she bought lingerie, a dress, bed linen and some shoes. She was captured browsing there on CCTV.

And that, according to all official records, was the last trace of her. Many versions of the rest of her story have been told, among them that she used the cover of 9/11’s chaos to escape her increasingly complicated life; that she was killed at the World Trade Center, heroically helping those injured by the terrorist attacks; that she met with foul play on a night out.

Not a single one of those theories are supported by anything but the flimsiest scraps of evidence.

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